The alkaline porphyry associated Yao?an gold deposit, Yunnan, China: rare earth element and stable isotope evidence for magmatic-hydrothermal ore formation

Abstract

The Yao’an vein-type gold deposit is located in the Ailaoshan-Jinshajiang alkaline intrusive belt, Yunnan Province, China, and is associated both in time and space with 33.5±1.0-Ma-old alkaline intrusions. The gold mineralization is associated with potassic wall-rock alteration. The REE distribution patterns of secondary K-feldspar are generally similar to those of the igneous perthite but with about seven times higher total REE abundances. The alteration is ascribed to a high-REE magmatic fluid derived from the Yao’an alkaline intrusive suite. The hydrothermal Yao’an gold deposit formed during two gold-bearing stages, i.e. a sulfide (pyrite) stage and a sulfide-oxide stage (pyrite-specularite). The REE abundance of early stage I pyrite is relatively high with strong enrichment in LREE, (La/Yb)n of 40–290, generally positive Eu anomalies (Eu/Eu*=0.86–1.55), and REE patterns very similar to those of secondary feldspar. In contrast, the later stage II pyrite has much lower REE concentrations and lower (La/Yb)n of 5.5–11.8, Eu/Eu* of 0.49–0.76, and flat chondrite-normalized spidergram patterns. The stage I pyrite has δ34S in the range of –2.2 to +3.2‰, and overlaps with regionally distributed pyrite in least-altered syenite porphyry. In contrast, stage II pyrite has much higher δ34S values between +7.8 and +16.5‰. Carbon isotope data for four samples from stage II revealed δ13C (PDB) values between –6 and –8‰. These stable isotope and REE data suggest that magmatic fluids of the alkaline intrusions caused both potassic alteration and stage I sulfide mineralization. The system evolved from stage I to stage II mineralization by influx of meteoric fluids with relatively heavier sulfur, although calcite carbon isotope data suggest that the CO2 remained dominantly of magmatic origin.